1. Field of the Invention
The present invention relates to technology for controlling rotation of a rotor, and in particular, to a motor drive circuit which controls rotation of a motor including a stator having a plurality of coils and a rotor that is magnetic.
2. Description of the Related Art
A brushless direct-current motor is used for rotating a disk in electronic devices using disk-type media, such as portable CD (Compact Disc) devices, DVD (Digital Versatile Disc) devices, and the like. In general, the brushless direct-current (DC) motor is provided with a rotor having a permanent magnet, and a stator having a plurality of phase coils connected in a star configuration; the coils are excited by controlling current supplied to the coils, and the rotor is driven by rotation relative to the stator. The brushless DC motor is generally provided with a sensor such as a Hall element, optical encoder, or the like, in order to detect rotation position of the rotor, and a current supplying each phase coil is switched in accordance with a position detected by the sensor, to give an appropriate torque to the rotor.
In order realize a smaller motor, a sensorless motor has been proposed, which detects rotation position of the rotor without using the sensor, such as the Hall element or the like (for example, refer to Patent Documents 1 to 3). By measuring electrical potential at midpoint wiring of the motor (referred to below as midpoint voltage), for example, the sensorless motor monitors back electromotive voltage (back electromotive force) that occurs in the coils, and by detecting a zero-cross point equal to the midpoint voltage, obtains positional information.
In driving this type of sensorless motor, technology is known by which, using a Pulse Width Modulation (referred to below as PWM) method, current flowing in the phase coils is controlled, torque is adjusted, and control is gently performed to have a sine waveform or arch form.
Patent Document 1: Japanese Patent Application, Laid Open No. H03-207250
Patent Document 2: Japanese Patent Application, Laid Open No. H10-243685
Patent Document 3: Japanese Patent Application, Laid Open No. H11-75388
When the PWM method is employed, a drive current flows intermittently in the coils of the motor, according to signal level of a pulse signal. FIGS. 1A and 1B show waveforms of back electromotive voltage in motor driving by the PWM method. FIG. 1A shows a case in which a zero-cross point occurs during an ON time-period Ton, and FIG. 1B shows a case in which the zero-cross point occurs during an OFF time-period Toff. FIG. 1A shows back electromotive voltage Vu and midpoint voltage Vcom that occur in a phase coil during driving, in order from the top. A pulse signal Spwm (not shown in the figure) alternately and repeatedly has a high level and a low level, and, for example, a current flows in a coil during the ON time-period Ton at the high level, and the coil current is interrupted during the OFF time-period Toff at the low level. As a result, when pulse width modulation is carried out, as shown in FIG. 1, the back electromotive voltage Vu that occurs in the phase coil during driving has a voltage level which has meaning only during the ON time-period Ton when the drive current is flowing, and the back electromotive voltage Vu has a voltage close to 0 V, during the OFF time-period Toff when the drive current is not flowing. The back electromotive voltage Vu is compared with the midpoint voltage Vcom by a comparator, and at a point in time when the two voltages intersect, a zero-cross is detected. Therefore, conventionally it has been possible to perform detection of a zero-cross point only during the ON time-period Ton, but there has been a problem in that detection of the zero-cross point could not be performed during the OFF time-period Toff.
That is, if the zero-cross point occurs during the ON time-period Ton, as shown in FIG. 1A, detection is performed immediately at a timing when the zero-cross point occurs. On the other hand, in cases in which the zero-cross point is taken to occur during the OFF time-period Toff, since voltage comparison cannot be performed at that point in time, it has been necessary to wait for the next ON time-period Ton, in order to actually detect the zero-cross point, a time lag τ occurs, causing rotational irregularity or the like.